The techniques used in the fabrication of integrated circuits and flat panel displays include film deposition processes and etching or stripping processes. These processes are generally performed in a processing reactor containing a plasma which is formed by the application of RF or microwave power to a process gas in the reactor chamber. When films are formed or etched, it is important that the deposition or etching processes be controlled to deposit a film of the correct thickness or etch a film to the corrected depth.
Deposition of silicon dioxide (SiO.sub.2) is an important process for insulating between metal layers on integrated circuits or for processing devices such as flat panel displays. Determining the deposition rate of the SiO.sub.2 is important to ensure that the SiO.sub.2 film deposited on the substrate is of the desired thickness. One technique for obtaining a particular film thickness includes repeatedly depositing films on successive substrate samples until an appropriate film thickness is achieved and determining the period of time required to achieve the desired film thickness. However, this technique for obtaining a proper film thickness is time-consuming and relatively inaccurate. Accordingly, optical endpoint monitoring systems have been developed to monitor deposition processes through a window in the process chamber.
It is also important to monitor etching process endpoints in plasma chambers to determine when etching has been performed to a proper etch depth. Typically, endpoints of an etching process in plasma chambers are monitored by analyzing light emissions of plasmas in the plasma chamber or laser illumination reflected from the substrate through a window in the chamber. For example, U.S. Pat. No. 4,615,761 involves monitoring the quantity of radiation corresponding to a selected one of the plasma reaction products to ascertain when an etching reaction is finished. Also, U.S. Pat. No. 5,045,149 discloses a method and apparatus for detecting the endpoint of a process of etching a first material which has been formed over a second material. According to this method the optical emission intensity of the plasma etch process is simultaneously monitored by a positive filter and a negative filter generating first and second signals, respectively. The first and second signals are combined to yield a combined signal which is monitored for a change indicative of the first material having been etched away exposing the second material.
In addition to monitoring endpoints of plasma etching processes, endpoints of plasma deposition processes are also monitored by measuring optical emission intensities. For example, U.S. Pat. No. 5,450,205 discloses monitoring of an etching or deposition process by a charge coupled device (CCD) camera during plasma processing. Plasma emission from the plasma processing chamber or laser illumination reflected from the substrate exhibit different optical characteristics which are measured to monitor etching or deposition process endpoints.
It is also desirable to accomplish the objective of monitoring endpoints in an in-situ clean process. Plasma reactors accumulate reaction byproducts on their interior chamber surfaces as films are deposited or etched in the chamber. These byproducts must be periodically cleaned off the inside walls of the reactor before the accumulated particles on the sidewalls interfere with and contaminate the deposition or etching process. One method of cleaning a plasma reactor is to create a fluorine plasma from NF.sub.3 to chemically react with the SiO.sub.2 within the chamber. However, the fluorine plasma corrodes the reactor over time and is very expensive. Therefore, it is desirable to use a minimum amount of NF.sub.3 to clean the reactor adequately. Techniques used to determine when a cleaning process is complete include the monitoring of optical emissions from the plasma within the reactor. The monitoring of optical emissions allows the in-situ clean process to be ended as soon as possible after all undesirable SiO.sub.2 material has been removed from the inside of the reactor to avoid damage to the reactor, minimize expense, and reduce reactor downtime.
Accordingly, it is desirable to monitor many different plasma processes including deposition, etching, and cleaning by optical endpoint detection through a window. However, plasma reactor gases, byproducts, or other particles tend to become deposited onto the window during processing in the plasma process chamber partially or completely blocking the light passing through the window. Such deposits on the window adversely effect the endpoint detection system. Accordingly, it would be desirable to maintain the window or optical viewport clear of any plasma reactor gases, byproducts, or other particles which adhere to and contaminate the window.